status: keep second-ambiguous mtimes during fixup

Now that we support the feature, we can keep "second ambiguous" mtime during the fixup phase.
These are the mtime that would be ambiguous if we did not had sub-second précions.

See the v2 format documentation for details.

Differential Revision:
11 jobs for branch/default in 41 minutes and 25 seconds (queued for 1 second)
Name Stage Failure
windows-py3-pyox Tests
adding extra file python3.dll to .
adding extra file python39.dll to .
adding extra file vcruntime140.dll to .
adding extra file vcruntime140_1.dll to .
installing files to C:\Users\heptapod-runner\builds\uxZezqu4\0\mercurial\mercurial-devel\build\pyoxidizer\x86_64-pc-windows-msvc\release\app
error: error calling build(): Runtime(RuntimeError { code: "TUGGER_FILE_MANIFEST", message: "Access is denied. (os error 5)", label: "" })
"make": *** [pyoxidizer] Error 1
Cleaning up file based variables
ERROR: Job failed: exit status 1